1. Field of the Invention
The present invention relates to self-cleaning surfaces and to a process for their production. In particular, the present invention relates to self-cleaning surfaces that remain self-cleaning for prolonged periods of time despite natural erosion.
2. Discussion of the Background
Articles with surfaces that are extremely difficult to wet have a number of commercially significant features. The feature of most commercial significance here is the self-cleaning action of low-wettability surfaces, since the cleaning of surfaces is time-consuming and expensive. Self-cleaning surfaces are therefore of very great commercial interest. The mechanisms of adhesion are generally the result of surface-energy-related parameters relating to interaction of the two surfaces that are in contact. The systems generally attempt to reduce their free surface energy. If the free surface energies between two components are intrinsically very low, it can generally be assumed that there will be weak adhesion between these two components. The important factor here is the relative reduction in free surface energy. In pairings where one surface energy is high and one surface energy is low the crucial factor is very often the opportunity for interactive effects. For example, when water is applied to a hydrophobic surface it is impossible to bring about any noticeable reduction in surface energy. This is evident in that the wetting is poor. The water applied forms droplets with a very high contact angle. Perfluorinated hydrocarbons, e.g. polytetrafluoroethylene, have very low surface energy. There are hardly any components that adhere to surfaces of this type, and components deposited on surfaces of this type are in turn very easy to remove.
The use of hydrophobic materials, such as perfluorinated polymers, for producing hydrophobic surfaces is known. A further development of these surfaces consists in structuring the surfaces in the μm to nm range. U.S. Pat. No. 5,599,489 discloses a process in which a surface can be rendered particularly repellent by bombardment with particles of an appropriate size, followed by perfluorination. Another process is described by H. Saito et al. in “Surface Coatings International” April 1997, pp. 168 et seq. Here, particles made from fluoropolymers are applied to metal surfaces, whereupon a marked reduction was observed in the wettability of the resultant surfaces with respect to water, with a considerable reduction in tendency toward icing.
U.S. Pat. No. 3,354,022 and WO 96/04123 describe other processes for reducing the wettability of articles by topological alterations in the surfaces. Here, artificial elevations or depressions with a height of from about 5 to 1000 μm and with a separation of from about 5 to 500 μm are applied to materials which are hydrophobic or are hydrophobicized after the structuring process. Surfaces of this type lead to rapid droplet formation, and as the droplets roll off they absorb dirt particles and thus clean the surface.
This principle has been borrowed from the natural world. Small contact areas reduce Van der Waal's interaction, which is responsible for adhesion to flat surfaces with low surface energy. For example, the leaves of the lotus plant have elevations made from a wax, and these elevations lower the contact area with water. WO 00/58410 describes these structures and claims the formation of the same by spray-application of hydrophobic alcohols, such as 10-nonacosanol, or of alkanediols, such as 5,10-nonacosanediol. The separations of the elevations in the structures are in the range from 0.1 to 200 μm and the heights of the elevations are from 0.1 to 100 μm. However, no information is given concerning the shape of the elevations. A disadvantage here is that the self-cleaning surfaces lack stability, since the structure is removed by detergents.
Another method of generating easy-clean surfaces has been described in DE 199 17 367 A1. However, the coatings, based on fluorine-containing condensates, are not self-cleaning. Although there is a reduction in the area of contact between water and the surface, this is insufficient.
EP 1 040 874 A2 describes the embossing of microstructures and claims the use of structures of this type in analysis (microfluidies). A disadvantage of these structures is their unsatisfactory mechanical stability.
Processes for producing the structured surfaces are likewise known. Besides the precision-casting reproduction of these structures by way of a master structure, by injection molding, or by embossing processes, there are other known processes which utilize the application of particles to a surface, e.g. in U.S. Pat. No. 5,599,489. Common features of all casting processes are that the self-cleaning behavior of the surfaces can be described by way of a very high aspect ratio, and that the structures have three-dimensional periodicity.
High aspect ratios in three-dimensional space, i.e. objects which are tall and narrow and stand in isolation, are difficult to produce industrially and have low mechanical stability.
There has been much relatively recent work concerned with the three-dimensional structuring of surfaces, an example being U.S. Pat. No. 6,093,754, where a three-dimensional structure is achieved by way of multiple printing of the surface, some of the printing inks repelling the next layer so that a structure is formed.
C. Bernard and D. Lebellac describe in FR 2792003 A1 a process for producing structured surfaces which are both water-repellent and oil-repellent, by way of vacuum deposition, using a CVD technique. These layers, too, have insufficient mechanical stability.
Reducing the aspect ratio generally also increases the stability of the layers. For example, in “Physikalische Blätter” 56 (2000), No. 4, 49 et seq. Frank Burmeister describes a process for obtaining nanostructures by means of capillary forces. It is emphasized that structures from a few atomic layers up to the particle radius can be varied in such a way that it is also possible to generate structures with an aspect ratio>1. However, it is also emphasized that processes of this type are only useable for relatively small areas, since otherwise stress cracks can form in the drying process.
In “Advanced Materials”, 2001, 13, No. 1, pp. 51 et seq., Hideshi Hattore describes a process for electrostatic coating, emphasizing that self-organization of the particles occurs if the surface to be coated and the particles themselves carry opposite electrical charges. However this process does not generate aspect ratios>1, and these layers are therefore merely antireflection layers.
An interesting process for generating self-cleaning surfaces is described by Akira Nakajima, Langmuir 2000, 16, 5754–5760, where the structure is generated by subliming aluminum acetylacetonate. 2% of titanium dioxide also has to be added to the hydrophobic material in order to achieve self-cleaning properties. The cause of the effect here is certain to be the catalytic decomposition properties of the titanium dioxide in combination with light, rather than the structure and the hydrophobic properties.
DE 101 18 351 and DE 101 18 352 say that stable self-cleaning surfaces can be obtained by securing structure-formers having a fissured structure. If use is made here of hydrophobic structure-formers and hydrophobic carrier materials, these surfaces are to some degree mechanically stable and to some degree resistant to erosion by wind, weather and light. However, it is impossible to avoid ablation of the active layers, in particular when damage is caused by UV light. As is generally the case, attack by wind and weather leads to gradual smoothing of the surface and thus to fall-off in the self-cleaning effect. The self-cleaning action falls away asymptotically. The limiting value reached is dependent on the amount of residual structure remaining on the carrier, and the extent of hydrophobic properties and smoothness possessed by the resultant surface.
DE 199 44 169 A1 achieves self-cleaning surfaces by way of incipient erosion of an “outer layer”. In this process, the effect does not appear until erosion has occurred. Further erosion causes the self-cleaning effect to reduce or disappear entirely. There is no regeneration of the self-cleaning surface.